The PLC engineers I had contact with usually had an electrical engineering background. That's why they like PLCs in the first place with the ladder logic programming languages, grafcet and if they feel fancy a bit of structured text (assembly like) or structured control language (pascal like). They indeed did not know much about software security but a great deal about machine safety.
A real security nightmare are older OPC servers (OPC-DA) which is super reliant on DCOM. OPC is quite important to connect the PLCs to SCADA systems or 3rd party devices.